Multi-carrier signal transmitter and multi-carrier signal receiver

ABSTRACT

In transmitting first information necessary for gaining synchronism of transmission signal and second information which is the other information by determining a break of a single modulation unit with a simple structure and processing prior to Fourier transformation on the side receiving a signal transmitted as a multi-carrier signal, a transmission symbol stream is expanded on a frequency axis. With a predetermined frequency position (for example, 0 kHz) as reference, a transmission symbol stream symmetrical is generated on the frequency axis and then, the transmission symbol stream symmetrical on the frequency axis is Fourier-transformed and transmitted.

TECHNICAL FIELD

The present invention relates to a multi-carrier signal transmittingapparatus and a multi-carrier signal receiving apparatus preferable forapplication to a case of transmitting multi-carrier signals by radio.

BACKGROUND ART

Recently, OFDM (Orthogonal Frequency Division Multiplex: orthogonalfrequency division multiplexing) system has been employed as atransmission system highly resistant to multi-pass interference as wellas having an excellent frequency use efficiency. According to this OFDMsystem, a plurality of carriers (hereinafter referred to as sub-carrier)orthogonal to each other are disposed at every predetermined frequencyinterval within a single transmission band and data is distributed torespective sub-carriers and modulated for transmission. According tothis system, its transmission apparatus disposes transmission dataobtained in time series virtually on a frequency axis, allocatestransmission data to each sub-carrier, and orthogonally transforms it tomulti-carrier signals at the predetermined frequency interval by reversefast Fourier transformation or the like. A receiving apparatus thereofconverts received multi-carrier signal inversely to transmission time todata secured in time series so as to obtain reception data.

FIG. 1 is a diagram showing an example of the structure of a radiotransmission apparatus according to the OFDM system. Hereinafter, thestructure will be described. Here, a radio transmission apparatus 100comprises a video circuit 101 and a voice circuit 102 and the respectivecircuits 101, 102 carries out processing for encoding the inputted videosignal and voice signal. For example, the video circuit 101 performsencoding according to irreversible image compression encoding methodsuch as a processing for converting animation video signal to MPEG(Moving Picture Expers Group) system image data, a processing forconverting static image video signal to JPEG (Joint Photographing codingExperts Group) system image data. Or encoding with reversible imagecompression method like JBIG (Joint Bi-level Image Experts Group) ispermissible. A voice circuit 102 carriers out encoding based on the MPEGaudio method, CELP (Code Excited Linear Prediction) method, PCM (PulseCode Modulation) method or the like. In the meantime, the coded data maybe provided with ECC (Error Correcting Code) such as the Reed-Solomoncode, the turbo code and the like.

Video data outputted by a video circuit 101 and voice data outputted bythe voice circuit 102 are supplied to a mixing circuit 103 in which theyare converted to single-system data. After that, it is supplied to aninterleaver 104, in which interleave processing is carried out bychanging data arrangement to disperse bit series. Data interleaved bythe interleaver 104 is subjected to modulation processing by a modulator105. In this modulator 105, first, a preamble signal is inserted intothe bit sequence and next, as a primary modulation, for example, DQPSKmodulation (Differential Quadrature Phase Shift keying) is carried out.In the meantime, other modulation method than the DQPSK modulation maybe employed such as QPSK, BPSK, 8PSK, QAM and the like.

Data primarily modulated by the modulator 105 is supplied to a reversefast Fourier transformation circuit (IFFT circuit) 106 and as asecondary modulation, reverse Fourier transformation processing forconverting data disposed on time axis to data arrangement on thefrequency axis by arithmetic processing of reverse Fouriertransformation is carried out and further, window application processingis carried out by multiplying window data. If the reverse Fouriertransformation processing is carried out in this IFFT circuit 106, atransmission symbol stream disposed on the frequency axis virtually upto then is averaged so as to be transmission series. In the IFFTcircuit, each time when data of a predetermined unit is inputted,reverse Fourier transformation arithmetic processing is carried out forthat inputted data. In this specification, time for carrying out thearithmetic processing of this one unit is called single modulation time.

Output of the IFFT circuit 106 is supplied to a digital/analog converter107 so as to be converted to analog signal. After the conversion, theanalog signal is supplied to a high-frequency portion (RF portion) 108,in which high-frequency processing such as filtering, frequencyconversion are carried out so as to gain a transmission signal of apredetermined transmission channel. After that, it is transmitted byradio through an antenna 110. Processing timing in each circuit in theradio transmission apparatus 100 is controlled by a time base controller(TBC) 109.

FIG. 2 is a diagram showing a radio reception apparatus for receiving asignal transmitted from the radio transmission apparatus 100 shown inFIG. 1. Hereinafter, the structure thereof will be described. The radioreception apparatus 200 supplies a signal received by an antenna 201 toa high-frequency portion (RF portion) 202 so as to carry out suchreception processing as filtering and frequency conversion.Consequently, a reception signal of a predetermined channel is obtained.This reception signal is supplied to the analog/digital converter 203and converted to digital data. Reception series subjected to digitalconversion is supplied to a window detecting portion 204. This windowdetecting portion 204 carries out processing for detecting synchronismby detecting a break in data to be subjected to Fourier transformationbased on window data multiplied by the transmission system fromreception series.

Output of the window detecting portion 204 is supplied to the fastFourier transformation circuit (FFT circuit) 205 and transformationprocessing is carried out, in which Fourier transformation action iscarried out at the timing of the break in data detected by the windowdetecting portion 204 and data on the frequency axis is converted todata arrangement on time axis by the arithmetic processing of theFourier transformation. The reception series Fourier transformed issupplied to a decoder 206, in which decoding processing for returningconversion processing applied at the time of transmission such as theDQPSK modulation is carried out so as to generate a reception symbolstream.

This reception symbol stream is supplied to a deinterleaver 207, inwhich deinterleave processing for returning bit series dispersed byinterleave processing at the time of transmission to its original dataarrangement is carried out so as to obtain reception encoding bitseries. This reception encoding bit series is supplied to a viterbidecoder 208 and converted to reception information bit series by viterbidecoding processing. Video information in the converted receptioninformation bit series is supplied to a video circuit 209 and voiceinformation is supplied to a voice circuit 210.

In the video circuit 209, data encoded by the video circuit 101 of thetransmission system is decoded so as to obtain transmitted video data.In the voice circuit 210, data encoded by the voice circuit 102 of thetransmission system is decoded so as to obtain transmitted voice data.Processing timing in each circuit in the radio reception apparatus 200is controlled by a time base controller (TBC) 211.

With the above described structure, transmission and reception of theOFDM system signal are carried out. The primary modulation by themodulator 105 at the time of transmission is a modulation system inwhich the phase of carrier is changed in discrete manner depending ontransmission data, so that it has a large advantage in frequencyapplication efficiency. Because, in the reverse Fourier transformationprocessing in the IFFT circuit 106, the bit series disposed on thesubcarrier is averaged on time axis, it has such a large advantage thatit is highly resistant to interfering wave such as fading and shadowing.

However, on the side receiving such multi-carrier signal, respective bitseriess cannot be decoded until Fourier transformation processing in theFFT circuit 205 is carried out. If a break for one modulation(hereinafter referred to break) is not recognized properly when the FFTcircuit 205 executes the Fourier transformation processing at the timeof reception, accurate bit series cannot be decoded.

To achieve proper Fourier transformation action in the FFT circuit, itis necessary to determine the break depending on a power level oftransmission data because the break (hereinafter referred to as break)of one modulation time cannot be determined from data received in acircuit on a prestage of the Fourier transformation circuit (windowdetecting portion 204 in FIG. 2). Ordinarily, for the known preamblesignal contained in the transmission data, correlation in power level isobtained. In order to increase the accuracy of correlation value to beobtained here, calculation is necessary without reducing the bit widthof each channel. For the reason, there is a problem that the scale of acircuit for detecting the correlation is increased.

DISCLOSURE OF THE INVENTION

The present invention has been achieved in views of problems in theabove described radio transmission of the multi-carrier signal and anobject of the present invention is to enable demodulation by determininga break of one modulation unit with a simple structure or processingprior to Fourier transformation on the side receiving a signal to betransmitted as a multi-carrier signal.

According to a first invention, there is provided a multi-carrier signaltransmission apparatus for transmitting a signal in which firstinformation necessary for gaining synchronism of transmission signal isdisposed at a predetermined interval in second information which is theother information, comprising: data arrangement means for arranging thefirst information and the second information; first modulation means forgenerating a transmission symbol stream by modulating data created bythe data arrangement means; symbol generating means for expanding atransmission symbol stream generated by the first modulation means onfrequency axis so as to generate the transmission symbol streamsymmetrical on the frequency axis; and second modulation means forconverting the transmission symbol stream symmetrical on the frequencyaxis generated on the symbol generating means by reverse Fouriertransformation.

Consequently, first information necessary for gaining synchronism iscontained in transmission symbol disposed symmetrically with respect toa reference position on frequency axis. As a result, the side receivinga signal transmitted from this apparatus is capable of extracting onlyany one of real number portion and imaginary number portion of the firstinformation.

According to a second invention, there is provided a multi-carriersignal transmission apparatus according to the first invention whereinthe data arrangement means disposes the first information and the secondinformation alternately.

Consequently, when self correlation of the first information is carriedout in a reception circuit, a more highly accurate correlation value canbe produced.

According to a third invention, there is provided a multi-carrier signaltransmission apparatus according to the first invention wherein with asymbol at the reference frequency position of the transmission symbolstream as the center, the symbol generating means expands respectivesymbols of the transmission symbol stream other than that symbolsymmetrically on the frequency axis.

Consequently, symbols are arranged symmetrically on the frequency axiscentering on a frequency position which serves as reference like 0 kHzand the like, so that the symbols can be expanded symmetrically on thefrequency axis favorably.

According to a fourth invention, there is provided a multi-carriersignal reception apparatus for receiving multi-carrier signal includingfirst information necessary for gaining synchronism of transmissionsignal and second information, comprising: memory means for memorizingany one of a real number portion and an imaginary number portion in thefirst information; delay means for delaying a received symbol stream bya predetermined time; a filter portion for extracting the firstinformation using a reception symbol stream delayed by the delay meansand a reception symbol stream not delayed; a correlator for gainingcorrelation between an output of the filter portion and the firstinformation of the real number portion or imaginary number portionmemorized in the memory means; and determining means for detectingsynchronism depending on a peak position or a correlation value of thecorrelator.

Consequently, when symbols disposed symmetrically with respect to thereference position on the frequency axis are received, by gainingcorrelation between first information extracted from that receivedsymbol and preliminarily prepared first information, only a correlationvalue of any one of the real number portion and the imaginary numberportion is detected when its correlation is detected. Thus, as the firstinformation which should be prepared preliminarily for detection of thecorrelation within the reception apparatus, only any one of the realnumber portion and the imaginary number portion has to be prepared. As aresult, the amount of the first information prepared in the receptionapparatus can be reduced correspondingly and a processing amount fordetecting the correlation can be reduced, so that information forgaining synchronism contained in the received multi-carrier signal canbe detected prior to Fourier transformation with simple structure andsimple processing.

According to a fifth invention, there is provided a multi-carrier signalreception apparatus according to the fourth invention wherein whenprocessing time of a single unit for Fourier-transforming themulti-carrier signal is a single modulation time, a predetermined timeto be delayed by the delay means is set to ½ a single modulation time.

Consequently, only the first information necessary for gainingsynchronism can be extracted easily from the transmission symbol inwhich the first information and the second information are arrangedalternately.

According to a sixth invention, there is provided a multi-carrier signaltransmission apparatus for transmitting first information necessary forgaining synchronism of transmission signal and second information whichis the other information as the multi-carrier signal, comprising: firstmodulation means for generating a transmission symbol stream by thefirst information and a transmission symbol stream by the secondinformation selectively; and symmetrical transmission symbol streamgenerating means in which a transmission symbol stream based on thefirst information generated by the first modulation means is thetransmission symbol stream expanded symmetrically on frequency axis withrespect to a predetermined frequency position.

Consequently, transmission of the transmission symbol stream composed ofthe first information necessary for gaining synchronism and transmissionof the transmission symbol stream composed of the second informationwhich is the other information can be carried out selectively. Thus,when transmitting an asynchronous packet, it is possible to transmit thesymbol stream composed of the first information symmetrically on areference frequency position at a head slot and then transmit just thesymbol stream composed of the second information at a next slot, therebyenabling information necessary for gaining synchronism to be transmittedeffectively.

According to a seventh invention, there is provided a multi-carriersignal transmission apparatus according to the sixth invention whereinwith a symbol at the reference frequency position of the transmissionsymbol stream as the center, the first modulation means expandsrespective symbols of the transmission symbol stream other than thatsymbol symmetrically on the frequency axis.

Consequently, with a reference frequency position like 0 kHz as thecenter, symbols are arranged symmetrically on the frequency axis so thatthe symbols can be expanded symmetrically on the frequency axisfavorably.

According to an eighth invention, there is provided a multi-carriersignal reception apparatus for receiving the first information necessaryfor gaining synchronism of a transmission signal and second informationwhich is the other information, comprising: memory means for memorizingthe first information; correlator for gaining correlation between thereceived symbol stream and the first information of the real numberportion or imaginary number portion memorized in the memory means; anddetermining means for detecting synchronism depending on a peak positionof a correlation value of the correlator.

Consequently, when receiving the symbol stream composed of the firstinformation, a correlation value is detected from only any one of thereal number portion and the imaginary number portion contained in thatreception symbol. Therefore, as the first information to bepreliminarily prepared for detection of the correlation within thereception apparatus, only any one of the real number portion and theimaginary number portion has to be prepared, so that the amount of thefirst information prepared within the reception apparatus can be reducedcorrespondingly and further, the processing amount for detecting thecorrelation can be reduced. Thus, information for gaining synchronismcontained in the received multi-carrier signal can be detected prior toFourier transformation by simple structure and simple processing.

According to a ninth invention, there is provided a multi-carrier signalreception apparatus according to the eighth invention wherein the memorymeans memorizes only any one of the real number portion and theimaginary number portion in said first information.

Consequently, information for gaining synchronism contained in thereception symbol can be detected easily using storage information havinga small information amount.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing an example of a transmission systemfor a multi-carrier signal.

FIG. 2 is a block diagram showing an example of a reception system forthe multi-carrier signal.

FIG. 3 is a block diagram showing an example of a transmission systemaccording to a first embodiment of the present invention.

FIG. 4 is an explanatory diagram showing an example of a transmissionsymbol stream according to a first embodiment of the present invention.

FIG. 5 is an explanatory diagram showing an example of the transmissionsymbol stream expanded according to the first embodiment of the presentinvention.

FIG. 6 is a block diagram showing an example of a preamble signaldetecting structure according to a first embodiment of the presentinvention.

FIG. 7 is a waveform diagram showing an example of waveforms in Ichannel and Q channel of preamble contained in an expanded transmissionsymbol stream.

FIG. 8 is an explanatory diagram showing an example of a transmissionstate according to a second embodiment of the present invention.

FIG. 9 is an explanatory diagram showing an example of a transmissionsymbol stream according to a second embodiment of the present invention.

FIG. 10 is an explanatory diagram showing an example of the transmissionsymbol stream expanded according to the second embodiment of the presentinvention.

FIG. 11 is a block diagram showing an example of a preamble signaldetecting structure according to the second embodiment of the presentinvention.

FIG. 12 is a flow chart showing an example of preamble signal detectingprocessing according to the second embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, a first embodiment of the present invention will bedescribed with reference to FIGS. 3–7.

In this embodiment, the present invention is applied to a case ofcarrying out radio transmission of a multi-carrier signal. FIG. 3 showsan example of the structure of a transmission apparatus of thisembodiment. Hereinafter, the structure will be described. A radiotransmission apparatus 100′ comprises a video circuit 101 and a voicecircuit 102, and the respective circuits 101, 102 carry out processingfor encoding an inputted video signal and voice signal. Video dataoutputted by the video circuit 101 and voice data outputted by the voicecircuit 102 are supplied to a mixing circuit 103 so as to turn tosingle-system data. After that, this data is supplied to an interleaver104, which carriers out interleave processing in which bit series isdiffused by changing an data arrangement. Data interleaved by theinterleaver 104 is modulated by a modulator 105 for the purpose oftransmission. This modulator 105 inserts a preamble signal into a bitseries and then carriers out DQPSK modulation (or modulation by othermodulation methods such as QPSK, BPSK, 8PSK QAM) as the primarymodulation so as to obtain a symbol stream as modulation output.

FIG. 4 shows an example of the symbol stream outputted by the modulator105. In FIG. 4, the frequency axis is indicated virtually in the form ofan abscissa axis, in which 24 pieces of symbols are expressed in a unit.This unit of the symbol column is modulated to a multi-carrier signal bymeans of a circuit described later. In this structure, 24 pieces orsymbols S₁–S24 are disposed at predetermined frequency intervals(fs=4.17 kHz) between 0 kHz and 100 kHz, while the symbols S₁, S24 (thatis, a symbol located at 0 kHz and a symbol located at 100 kHz) locatedon both ends of the 24 symbols S₁–S24 serve for guard carriers so thatthey do not carry anything. Remaining 22 symbols S₂–S₂₃ are divided tosymbols for use for transmitting first information and symbols for usefor transmitting second information. The first information is a preamblesignal. The second information is data desired to be transmittedactually (for example, data obtained by encoding the video signal orvoice signal). In the meantime, a band from 0 kHz to 100 kHz is called abase band range.

As for disposition of the first information symbol and the secondinformation symbol, the first information symbol and the secondinformation symbol are disposed alternately. That is, as shown in FIG.4, the symbols S₂, S₄, S6, S8, S10, S12, S14, S16, S18, S20, S22 aredisposed as symbols for transmitting (modulating) the first informationS₁ and the symbols S₃, S₅, S₇, S₉, S₁₁, S₁₃, S₁₅, S₁₇, S₁₉, S₂₁, S₂₃ aredisposed as symbols for transmitting (modulating) the second informationD2. The symbol column shown in FIG. 4 is a symbol column to be processedin a single modulation time at the time of modulation to multi-carriersignal and here, called a symbol column in the unit of a singlemodulation.

According to this embodiment, data modulated primarily by the modulator105 is supplied to a symbol expanding portion 111. This symbol expandingportion 111 expands only the first information of the symbol column inthe unit of a single modulation symmetrically on the frequency axis.That is, when the symbol column disposed from 0 kHz to 100 kHz virtuallyon the frequency axis is inputted to the symbol expanding portion 111 asshown in FIG. 4, the symbols are expanded symmetrically on the frequencyaxis with the position of 0 kHz as a reference frequency position sothat this reference position serves as a central position.

FIG. 5 is a diagram showing an expanded state. With the symbol S₁disposed virtually at 0 kHz which is a reference position acting as aguard carrier in the center, the symbols S₂–S₂₄ are expandedsymmetrically vertically on the frequency axis (to the right and left ina state shown in FIG. 5), such that the symbols are disposed at apredetermined frequency interval fs in a range from −100 kHz to 100 kHz.Therefore, in the symbol developing portion 111, the symbol S₁ at thecenter position (reference position) is kept as it is while as of theother symbols S₂–S₂₄, even symbols S₂, S₄, S₆, S₈, . . . S₂₂ areexpanded in an opposite direction. That is, assuming that the frequencyposition of the symbol in which the first information is disposed in theorder of the frequency form −100 kHz is −f₁₁, −f₁₀, −f_(g), . . . −f₁,f₁, . . . f₉, f₁₀, f₁₁, as the symbols at frequency positions −f₁–−f₁₁lower than the reference position (0 Hz), the symbols S₂, S₄, . . . S₂₂are disposed in order from a higher frequency position. As the symbolsat frequency positions f1–f11 higher than the reference position, thesymbols S₂, S₄, . . . S₂₂ are disposed in order from a lower frequencyposition, so that they are inverted in terms of the right and leftdirection with 0 Hz as a border on the frequency axis.

Returning to the description of FIG. 3, a symbol stream outputted fromthe symbol developing portion 111 having such an arrangement is suppliedto a reverse fast Fourier transformation circuit (IFFT circuit) 106 andin a secondary modulation, data disposed on a time axis is subjected toreverse Fourier transformation processing so as to convert it to a dataarrangement pn the frequency axis by arithmetic processing of reverseFourier transformation. Further, the window application processing formultiplying this result by window data is carried out. If the reverseFourier transformation processing is carried out by this IFFT circuit106, the transmission symbol stream disposed virtually on the frequencyaxis up to here is averaged on a time axis so that it becomestransmission series. Data of a single modulation unit processed in asingle modulation time in this IFFT circuit 106 is a symbol columndisposed virtually in a range from −100 kHz to 100 kHz as shown in FIG.5.

An output of the IFFT circuit 106 is supplied to a digital/analogconverter 107 and converted to an analog signal. That converted analogsignal is supplied to a high-frequency portion (RF portion) 108 so as tocarry out high frequency processing such as filtering and frequencyconversion to obtain a transmission signal of a predeterminedtransmision channel. After that, it is transmitted by radio from anantenna 110. Processing timing in each circuit in the radio transmissionapparatus 100′ is controlled by a time base controller (TBC) 109.

Next, a radio receiving apparatus for receiving a multi-carrier signaltransmitted by radio under the structure shown in FIG. 3 will bedescribed. According to this embodiment, the basic structure forconducting receiving processing is the same as the structure of theradio receiving apparatus 200 shown in FIG. 2. The multi-carrier signalreceived here is subjected to a processing for converting symbolsdisposed on the frequency axis by the fast Fourier transformationcircuit (FFT circuit) to symbols on the time axis. A prestage circuit(window detecting portion 204 shown in FIG. 2) for transformationprocessing by the FFT circuit determines a break of reception data andthe determined break data is supplied to the FFT circuit (or circuit forcontrolling processing timing in the FFT circuit).

According to this embodiment, to determine a break of this receptiondata, a detection circuit having a structure shown in FIG. 6 is composedwithin a circuit disposed at a prestage of the FFT circuit so as todetect a preamble signal contained in the reception data. A circuitshown in FIG. 6 is a circuit to be incorporated in the window detectingportion 204 in the receiving apparatus shown in FIG. 2. If the receptionseries is obtained by an input terminal 11, a signal in which thisreception series is delayed by a delay circuit 12 and a signal in whichit is not delayed (that is, just a signal obtained in the input terminal11) are supplied to a subtractor 13 so as to carry out subtractionprocessing. A delay circuit 12 is a circuit for delaying the singlemodulation time of the reception series by only ½. Here, the singlemodulation time is assumed to be 240 μseconds and a delay processing iscarried out by the delay circuit 12 for 120 μseconds.

Carrying out of subtraction processing for the signal delayed by ½modulation time and the not delayed signal by means of the subtractor 13functions as a comb filter for extracting only the preamble signal fromsignal transmitted with the state shown in FIG. 5. The delay time in thedelay circuit 12 composing this filter is set based on the dispositionof the first information symbol and the second information symbol.

The preamble signal extracted by the subtractor 13 is supplied to ashift register 14. The shift register 14 is a register in which11-symbol data are set. A preamble buffer 15 stores the preamble data ofthe 11 symbols preliminarily. Correlation between data set in the shiftregister 14 and data stored preliminarily in the preamble buffer 15 isobtained by individual multipliers 16 a, 16 b, . . . 16 n for eachsymbol value. According to this embodiment, transmitted symbol data issymbol data subjected to DQPSK modulation. The DQPSK modulated symboldata is composed of I channel (real number portion which is an in-phasecomponent of an orthogonal modulated wave) and Q channel (imaginarynumber portion which is an orthogonal component of the orthogonalmodulated wave). Data of only the I channel which is a real numberportion is stored in the preamble buffer 15, so that only data of the Ichannel is compared in the multipliers 16 a, 16 b, . . . 16 k.

Then, output of the correlation value of the respective multipliers 16a–16 k is supplied to an accumulative adder 17, in which an electricpower level corresponding to 11 symbols are added accumulatedly andoutput of the added value is supplied to a determining portion 18. Thedetermining portion 18 carries out a processing for determining whetherthe electric power level obtained by accumulated addition is higher orlower than a threshold level set preliminarily. If it is determined thatit is higher than the threshold level, its determining output issupplied from a terminal 19 to a reception timing control means (circuitcorresponding to a time base controller 211 of FIG. 2) and then,processing timing in the FFT circuit and the like is controlled based onthe determined timing.

Here, a principle that the preamble signal can be detected by detectingcorrelation by means of a circuit shown in FIG. 6 will be described.Because modulation processing carried out on a received signal at thistime, namely modulation in the modulator 105 within the radiotransmission apparatus 100 is DQPSK modulation, it is data indicated bya position on a circle on the orthogonal coordinate axes formed byintersecting the I channel and the Q channel. In the modulator 105 onthe transmission side, as the second information shown in FIG. 4, anyone of these four points is selected. The first information isabsolutely modulated such that all are at the same phase positions.

Assuming that the basic waveform of each subcarrier in the I channel iscoe(2πft) and the basic waveform of each subcarrier in the Q channel issin(2πft), of two outputs (I channel and Q channel), transmission poweron one side (for example, I channel) is doubled because the component ofthe frequency fn and the component of the frequency −fn (for example,component of the frequency f1 and component of the frequency −f1 in FIG.5) strengthen each other in the output of the high frequency portion 108of the radio transmission apparatus 100′. If this is expressed by anequation, it can be expressed a follows.I component=Σ[cos(2πft)+cos{2π(−fn) t}]=2Σ[cos(2πft)]  [1]

On the other hand, in transmission power in the Q channel, its pluscomponent and minus component kill each other so that the transmissionpower becomes 0. If this is expressed by an equation, iL can beexpressed as follows.Q component=Σ[sin(2πft)+sin{2π(−fn)t}]  [2]

This is expressed by a waveform diagram in FIG. 7. An example of acarrier in the I channel of a subcarrier at a position −f1 shown in FIG.5 is shown in FIG. 7A and a carrier in the Q channel of a subcarrier inthe Q channel at a position f1 shown in FIG. 5 is shown in FIG. 7B. Byadding the waveforms shown in FIGS. 7A, 7B for each channel, it isevident that the levels in the I channel strengthen each other whilethey kill each other in the Q channel. Thus, with the structure shown inFIG. 6, by memorizing only the preamble signal of one channel (I channelhere) in the preamble buffer 15 and then obtaining correlation inreception power of the preamble signal, the preamble signal can bedetected accurately.

If comparing a processing at the detection circuit of FIG. 6 with aprocessing at the conventionally same preamble signal detecting circuit,the output of a filter (circuit corresponding to the subtractor 13 shownin FIG. 6) for extracting the preamble signal is expressed by A+jB wherethe value of the real number portion (I channel) is A while the value ofthe imaginary number portion (Q channel) is jB. On the other hand, whenthe real number value C and the imaginary number value jD of thepreamble signal are memorized in the preamble buffer, arithmeticoperation of this memorized value C'jD and the filter output value A+jBis expressed by a following equation.(A+jB)*(c+jD)=(AC−BD)+j(AD+BC)  [3]

Although the arithmetic operation of the equation [3] is a processing inthe conventional detecting circuit, in the detecting circuit of thisembodiment, memorized data in the preamble buffer 15 may be C alone. Ifcorrelation detecting processing is expressed, it is expressed by afollowing equation.(A+jB)*C=AC+jBC  [4]

Therefore, it is possible to omit two multiplication processing and twoaddition/subtraction processing for each preamble of a symbol. In thesignal structure described up to now, because there are provided 11symbols of the preamble signals are provided, the multiplicationprocessing of 22 pieces and addition/subtraction processing of 22 piecescan be omitted. Consequently, the structure of the preamble signaldetecting another circuit such as an adder so as to function as afilter.

Next, a second embodiment of the present invention will be describedwith reference to FIGS. 8–12.

In the embodiment also, the present invention is applied to a case forradio transmission of the multi-carrier signal. The basic structures ofits transmission apparatus and reception apparatus are the same as thosedescribed in the first embodiment. After the transmission symbol streamgenerated by the primary modulator at the time of transmission isexpanded in the symbol expanding portion, it is converted to themulti-carrier signal by means of the IFFT circuit (reverse fast Fouriertransformation circuit). This embodiment is an example suitable forcarrying out packet communication such as ATM (Asynchronous TransferMode) or the like. That is, as shown in FIG. 8, packets P1, P2, P3composed of time slots TS1, TS2, TS3 are transmitted as required. Here,a time slot has the length of a modulation time.

Processing capacity of the IFFT circuit provided in the transmissionapparatus is equal to capacity capable of processing signals (subcarrierinterval fs=4.17 kHz) of −200 kHz–200 kHz in terms of base band and whenthe width of 100 kHz is assumed to be a channel, it comes thatprocessing capacity corresponding to four channels is provided.

At the time slot TS1 at the head of each packets P1, P2, P3, thepreamble signal is sent and then, at the second and third time slots,other signals (signal corresponding to the second information in thefirst embodiment) are sent.

According to this embodiment, the transmission symbol stream primarilymodulated by DQPSK modulation in the modulator 105 within thetransmission apparatus has the structure shown in FIG. 9. That is, when24 pieces of symbols S₁–S₂₄ are disposed virtually on the frequency axisat 4.17 kHz, as for the frequency position, the frequency position ofthe symbol S₁ is 100 kHz while the frequency position of the symbol S₂₄is 200 kHz. In this example also, symbols S₁, S₂₄ (that is, symbollocated at 100 kHz and symbol located at 200 kHz) on both ends of 24symbols S₁–S₂₄ server for guard carriers, so that actually anything isnot transmitted. Remaining 22 symbols S₂–S₂₃ are symbols for use fortransmitting the first information (that is, preamble signal) throughthe head time slot TS1.

Then, the preamble signal having the structure shown in FIG. 9 isexpanded by the symbol expanding portion. At the time of expansion, with0 kHz as a reference position (that is, central position), processingfor expanding symmetrically on the frequency axis is carried out. FIG.10 is a diagram showing an example of symbols to be expanded in thiscase. As shown in the same Figure, the 24 symbols S₁–S₂₄ disposedvirtually from 100 kHz to 200 kHz are disposed from −200 kHz to −100kHz. Because they are disposed symmetrically with respect to 0 kHz, thearrangements of the symbols are reverse to each other.

Such expansion in the symbol expanding portion is carried out in onlythe time slot TS1 for transmitting the preamble signal which isinformation required to be determined before decoding in this exampleand symbols in a time slot period for transmitting other information(second information) are not expanded. Then, the symbol stream processedin this way is supplied to the IFFT circuit and subjected to reverseFourier transformation processing in which a time axis is converted to afrequency axis for every symbol in the modulation unit. A transformationoutput of the IFFT circuit is supplied to a high-frequency portion andtransmitted by radio in a predetermined transmission frequency band.

As a result of such transmission, only the I channel component of thepreamble signal is doubled as compared to a case where the transmissionpower level is not subjected to expansion processing. A principle thatthe transmission power level doubles is the same as the transmissionprinciple described in the first embodiment and therefore, a descriptionthereof is omitted.

Next, a radio receiving apparatus for receiving a signal transmitted inthis way will be described. The basic structure of the receptionapparatus is the same as the reception apparatus described in the firstembodiment. This embodiment has a different structure for detecting thepreamble signal contained in a reception signal. The AFT circuit fortransforming the multi-carrier signal is provided with a processingcapacity corresponding to four channels like the IFFT circuit providedin the transmission apparatus.

FIG. 11 shows the structure of the preamble signal detecting circuit ofa reception apparatus of this embodiment. Hereinafter, the structurethereof will be described. If a reception column is received by an inputterminal 21, this reception column is supplied to a shift register 22.The shift register 22 is a register in which data of 22 symbols is set.The preamble buffer 23 stores preamble data of 22 symbols preliminarily(FIG. 11 is a diagram in which part thereof is omitted and the registersand buffers are not expressed with 22 stages).

Then, correlation between data set in the shift register 22 and datapreliminarily accumulated in the preamble buffer 23 is obtained byindividual multipliers 24 a, 24 b, . . . 24 n for each symbol. Accordingto this embodiment, transmitted symbol data is symbol data subjected toDQSPK modulation. The DQPSK modulated symbol data is composed of Ichannel (real number portion which is an in-phase component of anorthogonal modulated wave) and Q channel (imaginary number portion whichis an orthogonal component of the orthogonal modulated wave). Data ofonly the I channel which is a real number portion is stored in thepreamble buffer 23, so that only data of the I channel is compared inthe multipliers 24 a, 24 b, . . . 24 n.

Then, output of the correlation value of the respective multipliers 24a–24 n is supplied to an accumulation adder 25, in which electric powerlevel corresponding to 22 symbols are added accumulatedly and an outputof the added values is supplied to a determining portion 26. Thedetermining portion 26 carriers out a processing for determining whetherthe electric power level obtained by accumulated addition is higher orlower than a threshold level set preliminarily. If it is determined thatit is higher than the threshold level, its determining output issupplied from a terminal 27 to a reception timing control means (circuitcorresponding to a time base controller 211 of FIG. 2) and then,processing timing in the FFT circuit and the like are controlled basedon the determined timing. When another time slot in the same pocket issubjected to transformation, the transformation processing is carriedout cyclically in the period of the modulation time from that determinedtiming. The principle that correlation detection is carried out with thestructure shown in FIG. 11 is the same as that mentioned about theequation [4] in the first embodiment.

By such reception processing, a break of the single modulation time canbe determined from reception data prior to Fourier transformation with asimple structure. Only by determining the reception power of any one ofthe real number portion and imaginary number portion composing thereception out continuously unlike the above described asynchronouscommunication any time slot period may be a time slot in which thepreamble signals described in this embodiment are disposed.

Further, it is permissible to change the base band for use upontransmitting the preamble signal depending on a communication state. Forexample, it is permissible that, when the transmission path state isgood, the base band of the time slot for transmitting the preamblesignal is in a range of −200 kHz to 200 kHz and when the transmissionpath state is not good, the base band of the time slot for transmittingthe preamble signal is in a range of −100 kHz to 100 kHz. In this case,the detecting circuit for the preamble signal in the reception apparatusmay be so constructed to correspond to both the signals.

If the base band of the time slot for transmitting the preamble signalis changed, this can be treated by carrying out a processing shown inFIG. 12, for example, in the symbol expanding portion in thetransmission apparatus. That is, after the transmission processing isstarted (step S101), application band information is obtained from atime base controller in this transmission apparatus (step S102). Next,the symbol stream corresponding to a single modulation time to betransmitted is accumulated in the buffer memory (step S103). Then, it isdetermined whether or not the frequency of a first symbol position(guard carrier in the above described example) of the transmissionsymbol stream is 0 kHz (step S104). This determination can be carriedout depending on the application band information obtained in, forexample, step S102.

If it is determined that the frequency of the first symbol position is 0kHz in this determination, the symbol S₂₄ to the symbol S₂ aretransmitted successively to the IFFT circuit (step S108). After that,the symbol S₁ to the symbol S₂₄ are transmitted successively to the IFFTcircuit (step S109). Then, with the symbol S₁ set at 0 kHz, themulti-carrier signal in which symbols thereof are expanded symmetricallyin a range of 31 100 kHz to 100 kHz is generated by reverse Fouriertransformation and transmitted, and the processing in this time slot isterminated (step S110).

Further, if it is determined that the frequency of the first symbolposition is not 0 kHz in step S104, the symbol S₂₄ to the symbol S₁ aretransmitted successively to the IFFT circuit (step S105) and afterwaiting in time equivalent to transmission time for about two channels(or transmitting symbol stream corresponding to its application band)(step S106), the symbol S₁ to the symbol S₂₄ are transmittedsuccessively to the IFFT circuit (step S107). A multi-carrier signal inwhich symbols thereof are expanded symmetrically in a range of −200 kHzto 200 kHz as shown in FIG. 10 is generated by reverse Fouriertransformation and transmitted, and then, the processing in this timeslot is terminated (step S1110).

In the meantime, the reason why the symbols to be transmitted to theIFFT circuit in step S108 are up to the symbol S₂ while the symbol S₁ isnot to be transmitted when symbols are expanded symmetrically in a rangeof −100 kHz to 100 kHz, is to have the respective symbols to be disposedsymmetrically with respect to the position of 0 Hz so that the symbolslocated at the respective symmetrical positions kill each other.

Although only the slot for transmitting the preamble signals is expandedsymmetrically for transmission according to this embodiment, it ispermissible to expand symbols symmetrically for transmission in a slotfor transmitting other information. Further, information to be disposedin a slot for transmitting other information the preamble signal may beprovided with error correction data before reverse Fouriertransformation is carried out. In this case, after Fouriertransformation is carried out on the side of the reception apparatus,error correction processing based on error correction data is carriedout.

In the processing for detecting the preamble signal within the receptionapparatus, its correlation detection processing may be carried out withsoftware as well as with a hardware circuit shown in FIG. 11.

1. A multi-carrier signal transmission apparatus for transmitting a signal in which first information necessary for synchronizing a transmission signal is disposed at a predetermined interval in second information, said apparatus comprising: data arrangement means for arranging said first information and said second information; first modulation means for generating a transmission symbol stream by modulating data created by said data arrangement means; symbol generating means for expanding said transmission symbol stream generated by said first modulation means on a frequency axis to generate a symmetrical transmission symbol stream that is symmetrical on said frequency axis; and second modulation means for converting said symmetrical transmission symbol stream generated by said symbol generating means by performing reverse Fourier transformation.
 2. The multi-carrier signal transmission apparatus according to claim 1, wherein said data arrangement means disposes said first information and said second information alternately.
 3. The multi-carrier signal transmission apparatus according to claim 1, wherein using a center symbol at a reference frequency position of said transmission symbol stream as a center, said symbol generating means expands symbols of said transmission symbol stream other than said center symbol symmetrically on said frequency axis.
 4. A multi-carrier signal reception apparatus for receiving a multi-carrier signal including first information necessary for synchronizing a transmission signal and second information, said apparatus comprising: memory means for storing one of a real number portion and an imaginary number portion in said first information; delay means for delaying a received symbol stream by a predetermined time period; a filter for extracting said first information using said reception symbol stream delayed by said delay means and a reception symbol stream that is not delayed; a correlator for correlating an output of said filter and one of said first information of said real number portion and said imaginary number portion stored in said memory means; and determining means for detecting a synchronism depending on a peak position of a correlation value of said correlator.
 5. The multi-carrier signal reception apparatus according to claim 4, wherein when a processing time of a single unit for Fourier-transforming the multi-carrier signal is a single modulation time, said predetermined time period used by said delay means is set at ½ said single modulation time.
 6. A multi-carrier signal transmission apparatus for transmitting first information necessary for synchronizing a transmission signal and second information as a multi-carrier signal, said apparatus comprising: modulation means for selectively generating a first transmission symbol stream by said first information and a second transmission symbol stream by said second information; and symmetrical transmission symbol stream generating means, wherein said transmission symbol stream based on said first information and generated by said modulation means is a symmetrical transmission symbol stream that is expanded symmetrically on a frequency axis with a predetermined frequency position as a reference.
 7. The multi-carrier signal transmission apparatus according to claim 6, wherein using a center symbol at said reference frequency position of said transmission symbol stream as a center said modulation means expands symbols of said transmission symbol stream other than said center symbol symmetrically on said frequency axis. 